1. Field of the Invention
The present invention relates to a display device mounted with a read function.
Further, the present invention also relates to an electric appliance using the display device mounted with a read function.
2. Related Art
In recent years, a display device using a light-emitting element as typified by an electroluminescent element or the like has been studied as an alternative to a liquid crystal display device. The display device is widely used by taking advantage of a high quality image due to a self luminous type, a wide viewing angle, a thin shape due to no backlight, lightweight, and the like.
There is a display device mounted with a read function which is a display device using the foregoing light-emitting element and which includes not only a light-emitting element but also an image pick-up element over a substrate (for example, refer to Patent document 1). The display device reads out information on an object to be read out in such a way that the image pick-up element receives light which is generated in the light-emitting element and which is reflected by the object to be read out to read out the information.
Patent Document 1: Unexamined Patent Publication No. 2002-176162
Light-emitting materials corresponding to each color required for color display have different current densities for obtaining predetermined luminance from each other. By using light-emitting materials corresponding to light's three primary colors as an example, the luminance of a green light-emitting element is higher than that of a red light-emitting element, the luminance of the red light-emitting element is higher than that of a blue light-emitting element. The foregoing light-emitting material causes variation in luminance of each color. Since white light emission is achieved by emitting all of three sub pixels of red, green, and blue corresponding to light's three primary colors, the white color cannot be clearly displayed depending on how the color of each sub pixel appears. For example, reddish white emission, bluish white emission, or the like is observed. When white balance becomes dulled, desired color cannot be obtained and an image expressed by a correct gray-scale becomes difficult to be displayed. There is an example of improving the white balance by correcting the number of gray-scale of a video signal inputted to three sub pixels of red, green, and blue to eliminate the variation of luminance of each color (for example, refer to patent document 2). Alternatively, there is an example of improving the white balance by varying power source potential given to each sub pixel instead of correcting a video signal.
Patent Document 2: Unexamined Patent Publication No. 2004-004708
According to the structure disclosed in the patent document 1, light may not be reflected by the object to be read out but be entered into the image pick-up element. This arises from the fact that light generated in the light-emitting element is reflected by an interface between a certain medium (for example, an insulating layer) and another certain medium between the light-emitting element and the object to be read out, and the reflected light is entered the image pick-up element. Only light reflected by the object to be read out should enter the light entering the image pick-up element. When such the extra light enters the image pick-up element, information on the object to be read out cannot be read out correctly.
According to the structure disclosed in patent document 2, a signal correction circuit for correcting the number of gray-scale of a video signal is required to be newly provided. When the number of ICs connected to the outside is increased as the signal correction circuit, the ICs interfere with the reduction of a size, a thickness, and weight. When a power source potential given to each sub pixel is varied to improve the white balance, it is required to generate some electrical potentials based on the power source potential supplied from a power source circuit; consequently, a circuit such as a level shifter is required to be provided. When the level shifter is not provided, various power source potentials are required to be provided from the outside; consequently, the number of pins of a panel is required to be increased.